bpo-43822: Improve syntax errors for missing commas (GH-25377)

diff --git a/Grammar/Tokens b/Grammar/Tokens
index 9de2da5..1f3e3b0 100644
--- a/Grammar/Tokens
+++ b/Grammar/Tokens
@@ -59,6 +59,7 @@
 ASYNC
 TYPE_IGNORE
 TYPE_COMMENT
+SOFT_KEYWORD
 ERRORTOKEN
 
 # These aren't used by the C tokenizer but are needed for tokenize.py